Cohort commander?

[Flavivs Aetivs]

By the Time of the Magister Militum Aetius' self-imposed Regency over Valentinian III Britain was it's own independent nation, not Roman.

Furthermore the Borders were defended - St. (Hygenius I think) of Noricum records that there was still a Garrison at Batavis, but the other Garrisons were destroyed or disbanded. Considering he's writing about 10 years after Aetius it can be theorized that Aetius or Majoran had the Danube Frontier (minus Pannonia Valeria and Pannonia Savia) garrisoned against the Iuthungi and Alemanni along the border, along with the Rugii and Amalic Goths.

Was there a shortage of men? Yes. Was the Roman Army any less effective because of it? No. Garrisons were moved around, Vexilliationes were created and positioned, the Garrisons were withdrawn to re-enforce the field army for major threats, and then put back, and so on and so forth.